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Question on power rating

  1. Aug 10, 2015 #1
    For example
    A computer with a power rating of 400w is switched on for 5 hours for a period of one week. What is the amount of electrical energy used?

    I calculated using E=PxT
    400w x (5hours x 7daysx 60mins x 60 seconds)

    But the answer is 14kwh
    Why is this so? I thought its power (in watts) x time (in seconds)?
  2. jcsd
  3. Aug 10, 2015 #2


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    Staff: Mentor

    They are different units for energy. Look up the conversion factor between joules (which is what you have calculated) and kilowatt-hours (which is what you want).

    Or, in your calculation of E = PT, use P in kilowatts and T in hours (instead of watts and seconds).
  4. Aug 10, 2015 #3
    oh. but then shouldn't the claculation be 0.4kw x 60secomds x 60 mins? i thought power is the electrical energy used per second ? why is it 0.4w x 7 x 5?
  5. Aug 10, 2015 #4
    or is it beacuse its kwh which means kw x h and not kw/h?
  6. Aug 10, 2015 #5


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    Science Advisor

  7. Aug 10, 2015 #6


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    Gold Member


    I assume that is 5 hours per day for one week.

    Energy (kWh) = Power (in KW) * Time (hours)

    = 0.4 * 5 * 7 = 14 kWh
  8. Aug 10, 2015 #7


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    Staff: Mentor

    Power is energy used per unit time. The unit of time can be (is) different for different units. In this case, it is -- you just have to look carefully at the unit and use it as specified.
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